1. Field of the Invention
The present invention relates to a control apparatus that is applied to a hybrid vehicle equipped with an engine in which an air-fuel ratio can be changed.
2. Description of Related Art
An engine that can execute lean combustion in which a target air-fuel ratio of the engine is set to a lean side in comparison with a theoretical air-fuel ratio has been well known in the art. Such an engine is equipped with an exhaust gas purification catalyst of an absorbing and reduction type for purifying nitrogen oxides in exhaust gas that is discharged due to the lean combustion. When the concentration of the nitrogen oxides absorbed in the exhaust gas purification catalyst exceeds a limit, the exhaust gas purifying capability reduces. Thus, a rich spike operation in which the air-fuel ratio is temporarily changed to a rich side is executed. Consequently, the nitrogen oxides absorbed in the exhaust gas purification catalyst are reduced, and the exhaust gas purifying capability of the exhaust gas purification catalyst is regained.
The rich spike operation involves the increase in the amount of fuel and thereby causes torque fluctuation of the engine. Japanese Patent Application Publication No. 11-190241 (JP 11-190241 A) discloses a control apparatus that corrects throttle opening during the rich spike operation in order to reduce the torque fluctuation.